This invention relates to a method of producing thermistor chips and more particularly to thermistor chips of the type having adhesively attached layers.
There have been demands to both miniaturize thermistor chips and to reduce their resistance values in order to lower the power loss due to a voltage drop. In view of these demands, Japanese Patent Publication Tokkai 6-267709 disclosed thermistor chips of a layered structure obtained by stacking up one on top of another a plurality of elements each having a positive temperature characteristic and having electrodes formed on both its main surfaces, attaching them by using an electrically conductive adhesive and connecting the individual elements in parallel. Thermistor chips with a low resistance value can be obtained by structuring them in this manner.
When thermistor chips with such a structure are to be produced, not only is it necessary to attach the individual elements with an adhesive such that their electrodes overlap each other but also to keep the electrodes on each element in a mutually insulated relationship. Thus, the structure had to be designed such that the conductive adhesive would not be applied to the area between the electrodes or an electrically insulating material had to be applied. Moreover, since it is necessary to stack up many elements with differently shaped electrodes, there was a high probability of increasing the production cost.
In view of the above, it may be considered to produce a thermistor by stacking up a plurality of elements through an electrically insulating material. Such a thermistor may be formed by preparing elements each having one ohmic electrode which covers almost entirely one of the main surfaces and extends over one of a side surfaces to the other main surface and another ohmic electrode which covers the other main surface almost entirely and extends over another side surface to the first main surface. These elements are stacked one on top of another in a main surface-to-main surface relationship through an insulating material such as a glass material in between. Outer electrodes are formed over the parts of these ohmic electrodes exposed on the side surfaces.
Thermistor chips of this type are advantageous in that the area over which the adhesive should be applied can be strictly controlled while an attempt is being made to reduce the resistance value. Moreover, since there is no need to use two kinds of adhesive agents, the structure can be made simpler. Since elements of only one kind are to be stacked up, the production cost can be reduced.
One of the methods for producing such thermistors would be to first prepare an ohmic electrode on mother substrates in the form of a green sheet, stacking them with an electrically insulating material inserted in between, cutting it into individual element and then subjecting them to a firing process. With such a method, however, electric charges move from the electrode material into the elements and generate voltage differences, and a barrier layer is generated between the electrode and the element. Since this functions as an electrical barrier, it works against the intended purpose of obtaining a thermistor with a reduced resistance.
According to a method considered for preventing the formation of such a barrier layer, ohmic electrodes are formed on the mother substrates which have already undergone a firing process, thereafter they are stacked with an insulating material in between and then a dicing blade or the like is used to cut it into individual elements. This method, however, is not economically feasible because the useful lifetime of a blade is not sufficiently long and this adversely affects the cost of production.
In view of the above, another method may be considered whereby ohmic electrodes are formed on a mother substrate having breaking grooves for making it easier to break it and after it is broken up along these grooves into individual elements, they are stacked up with an insulating material in between to form a thermistor chip having a layered structure. Such a method, however, could not produce dimensionally accurate products and hence the yield of "good" products was low because the stacking takes place after the mother substrate is broken up into elements.